Electrostatic sensor device and matrix

ABSTRACT

A system for electrostatically measuring the shape or pattern of objects including an array of sensor devices having the capability of responding, due to variable electrostatic coupling, to produce differentiated output signals when a sensor element forming part of each device is present and when it is not.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional No. 60/309,229filed Aug. 01, 2001.

FIELD OF INVENTION

The present invention relates to a system for measuring the location ofgauge pins, and the like, and more particularly, for the purpose ofthereby defining a shape or pattern in two or more dimensions, ofobjects or items that requires precise measurement. For example, such asystem could be used to measure the size and shape of a person's foot,but could be exploited in a wide array of other measurementapplications.

BACKGROUND OF THE INVENTION

One prior approach to the task of determining the shape of an object isto use a magnetic head captive inside a pin that is to be inserted intoa receptacle or opening in a circuit board. Once inside the openings, agroup of pins serves to outline the shape or pattern being measured.Then, when a Hall effect sensor, which is mounted on the circuit board,moves along the length of the pins it determines the magnets locationsand hence the pins locations.

The Hall effect technique, because it involves an expensive device formaking measurements for the purpose described and requires quite exactalignment of the parts involved has serious limitations in use.Accordingly, to overcome these limitations it is a principal object ofthe present invention to simplify the technique of measuring shapes andpatterns of objects.

SUMMARY OF THE INVENTION

Briefly stated, the primary aspect of the present invention is definedas follows: A system for measuring the shape or pattern of objectselectrostatically by having an array of sensor devices formed in acircuit board. Such sensor devices have the capability of respondingdifferentiatedly to input signals when a sensor element is present atparticular locations in the board and when it is not.

In a particular embodiment, the sensor device includes: A dielectricsubstrate material formed in two layers with a conducting plane betweenthe two layers; a sensing hole which penetrates the dielectric substratematerial from one outside surface to the other; a clearance hole in saidconducting plane such that the conducting plane does not obstruct thesensing hole; and a conducting ring surrounding the sensing hole on eachsurface of the dielectric substrate.

The foregoing and still further objects and advantages of the presentinvention will be more apparent from the following detailed explanationof the preferred embodiments of the invention in connection with theaccompanying drawings:

DESCRIPTION OF DRAWINGS

FIG. 1A is a top view of a sensor element formed in a dielectric printedcircuit board;

FIG. 1B is a side view of the sensor element;

FIG. 1C is a bottom view of the sensor element;

FIG. 1D is a diagram depicting a sensor element being sensed;

FIG. 2 is a block diagram of the electrostatic system for sensing thelocations of the active sensor devices;

FIG. 3 is an operational training diagram;

FIG. 4 is a schematic diagram of an array drive circuit for driving theX-Y array of sensor elements;

FIG. 5 is a schematic diagram of a pulse detector and clock generator,seen in FIG. 2, that are connected to the array drive circuit;

FIG. 6 is a schematic diagram of an amplitude reference generator seenin FIG. 2;

FIG. 7 is a schematic diagram of a sense amplifier and latch seen inFIG. 2;

FIG. 8A is a top view of the sensor element array;

FIG. 8B is a bottom view of the sensor element array;

FIG. 9 is a cross-sectional view of a sensing element in the form of apin; and

FIG. 10 is a profile view of the sensing pin.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the Figures of the drawing and particularly for themoment to FIG. 1, there will be seen an electrostatic system 10 formeasuring the shape or pattern of an object by sensing the location ofgauge pins or the like.

The system of sensor devices 12 is conveniently fabricated in a printedcircuit board matrix by employing standard fabrication facilities toproduce spaced holes or openings 14 for receiving the selectivelyinserted pins 16. Arranged with specially designed parts, the openings14 function as cooperative sensor elements with the movable elements inthe form of pins 16. Thus, an, etched metal drive ring 18 surrounds theupper end of each hole 14 in the circuit board (FIG. 1A), (the ringhaving, for example, 0.290 OD and 0.230 ID in inches. Likewise, anetched metal sense ring 20 surrounds the lower end of each hole 14 (FIG.1C). A metal shield layer 22 extends throughout the matrix 12 and issuitably provided with clearance holes 23 (0.240 ID) as indicated inFIG. 1B.

It will be seen that a trace connection 30 (FIG. 1A) for operation ofthe system extends from the ring 18 to a suitable pulse drive source(FIG. 4), while a trace connection 32 (FIG. 1C) extends to a suitablesensing circuit (FIG. 7). Each of the trace connections 30 and 32 wouldpreferably be formed to have a width of approximately 0.040 inches.

It will be appreciated that a sensor device 12 outputs a signal coupledby the capacitance between the two etched metal rings 18 and 20, ascoupled through the dielectric printed circuit board 13, and the senseelement, for example, in the form of a pin 16 (FIG. 10), placed in thesensing hole 14. For the dimensions given, the output signal will changeby about a ratio of 4 to 1 when the empty hole is filled with a metalcylinder 16A formed in a zone of the pin which has a clearance allaround of about 0.01 inch. This signal change can be sensed by asensitive amplifier or comparator, and the result stored in a memoryelement for later use, as will be shown and explained in an embodiment.

Operation of the electrostatic sensor system begins with a drive pulsebeing used to excite the etched metal ring 18 on the top surface of thesensor area and surrounding the hole or opening 14. The metal ring 18 isfabricated to have a small clearance, such as 0.015 inch, all around thehole drilled through the printed circuit board. This clearance preventsdirect contact between a conductive cylinder and the ring, for morerepeatable performance. The trace connection 30 on the top surface ofthe printed circuit board connects the ring 18 to a source of drivepulses, and to additional rings if desired. As seen in the side view(FIG. 1B) of the printed circuit board 13, the ring is fabricated over ametal ground plane (shield layer 22) buried in the middle of the printedcircuit board's thickness. The ground plane has a clearance hole 23,already noted, surrounding the sensing hole 14. The clearance hole isdrilled through the printed circuit board, so it cannot directly touchthe metal cylinder that is intended to be placed in the center of thesensing hole.

The bottom of the circuit board has a similar ring 20, used for sensingthe amount of drive signal coupled by the electrostatic capacitancebetween the metal cylinder and the drive signal ring. A smallcapacitance exists between the cylinder and each ring, passingpredominantly through the dielectric material and the air gap around thecylinder. A small amount of additional coupling occurs directly in theair above the ring to the cylinder on each side of the printed circuitboard. Note that the sense ring has a small clearance around the drilledhole, such as 0.015 inch as before. A trace connection 32 on the bottomsurface connects the ring to a sense amplifier for monitoring coupledpulses, and additional rings if desired.

When the center of the drilled sensing hole 14 is empty, there is stilla small amount of coupling through the hole between the drive and senserings. As an example, for the dimensions given and in a matrix array of36 columns by 16 rows, the residual signal with a 10-volt drive pulsewill be about 14 millivolts. If now the hole is filled with a 0.18 inchdiameter by 0.18 inch long metal cylinder on a dielectric support, thesignal received by the sensor ring will increase to about 63 millivolts.This signal increase of more than a factor of 4 is sufficient forreliable sensing of the presence of the metal cylinder in the sensinghole. When the sensing hole is filled with a dielectric cylinder of 0.18inch outside diameter, the signal received will increase to about 20millivolts. Thus the ratio of signal amplitudes between metal anddielectric cylinders is sufficient for reliable detection as before.Partial entry of the metal cylinder into the sensing hole 14 produces acorresponding analog response, and the subsequent signal processingamplifier will make the distinction of how much signal is needed toproduce a digital output. If the grounded shield plane in the center ofthe printed circuit board thickness is omitted, the stray couplingbetween the drive and sense rings increases greatly, and there isinsufficient signal variation for reliable sensing.

Referring now to FIG. 8, the sensor elements disclosed above can bearranged, as previously alluded to, in a two-dimensional array ormatrix, with the drive rings connected together along one array axis,and the sensing rings connected together along the other array axis.Designate the drive rings connected together as being in rows, and thesense rings connected together as being in columns. Then if M rows and Ncolumns are used, it is seen that one of the great advantages of thisdesign is that M times N sensor elements are accommodated by using onlyM drive pulse sources and N sense amplifiers. The sense elementsthemselves are simply etched patterns with holes drilled in the printedcircuit board, so they can be inexpensively mass-produced. No specialelectronic circuitry is required, as in the prior art, at each sensorlocation, so a substantial cost saving can result. For the case of 16rows and 36 columns, a total of 576 separate locations can be sensed andstill require only 52 independent signals and sets of circuitry. In thecase of interest here, the sensor elements are arranged in a uniformlyspaced rectangular array with a spacing of 0.315 inch between centers inboth row and column directions. With the ring outside diameter of 0.290inch, this gives a clearance space of 0.025 inch between the outsidediameters of rings in adjacent columns.

It should be noted, in connection with FIG. 8, that during experimentalmeasurements of prototype sensor elements, it was discovered that havingthe sensor rings closely spaced as above could lead to significantcross-coupling between adjacent columns. Specifically, if a signal wereinduced in one column by an active sensor, the columns adjacent to it oneither side could have induced signal amplitude of 3.7 percent of thesignal on the active column. Since the sensor operation is essentiallyanalog, and distance measurements will be made according to when thesensor output crosses a reference threshold, this could cause errors inthe measured distance values. This effect is possible on the sensor ringside of the sensor elements because the sense amplifier chosen foreconomic reasons permits significant signal voltage to be developed oneach column line. Stray coupling on the row drive side between rings isnot of importance since the row drive signals come from a low impedancesource, and the row lines are loaded with additional bypass capacitors.If an additional amplifier with a low input impedance were used for eachcolumn, then the parasitic coupling would have no effect as there wouldnot be any signal voltage on the column lines. The experimentally chosenmethod to reduce this undesirable effect is to place a thin, groundedmetal trace connection 30 on the surface of the printed circuit board.This trace connection is placed so as to run in between the ring edgesof adjacent sensor ring columns. With a clearance space of 0.025 inchavailable, a shielding trace reduces the induced parasitic coupling toless than 1.6 percent, which was judged acceptable for this application.

FIG. 2 is a block diagram of a sensing system. An external source ofdrive pulses 50 sends 10 volt amplitude pulses of 100 to 500 microsecondduration to the M rows of the sensor element matrix 13, one row at atime. The circuitry provided senses the presence or absence of aconducting cylinder, in the form of a metal zone 16A on sensing pin 16(FIG. 10), in each sensor location in a driven row, and outputs thatdata on the N digital column outputs 52. Operation begins when the pulsedetector determines that a pulse has occurred on one of the M input rowlines. The pulse detector triggers a clock pulse generator 56, and thetrailing edge of the clock pulse will be used to store the results ofthe sensing operation in a latch 58. An amplitude reference generator 60produces a voltage output VREF during the pulse input that is a preciseratio fraction of the drive pulse amplitude. Each column from the sensorarray 62 goes to an amplitude comparator, within the digital senseamplifier 64, which determines if the coupled signal from the sense ringon the driven row exceeds the VREF value. If the reference value isexceeded, the corresponding digital sense amplifier 64 output goes high.Otherwise, it stays low. Finally, the separately generated clock pulsestores the sense amplifier outputs from all the columns into a latch forlater use. The N columns digital outputs are held until the next row isdriven with a pulse.

A timing diagram for the sensing system operation is shown in FIG. 3.Operation begins with the drive pulse rising edge, which produces anoutput from the pulse detector 54 and the clock pulse generator 56. Thecolumn signal is proportionate to the drive pulse and couplingcapacitance at its start, and varies according to the material occupyingthe sensing hole. Because the coupling capacitance is very small(typically 0.1 picofarad), the row capacitance is typically 30picofarad, and the sense amplifiers 64 have a finite input impedance of470 kilohms, the column signal decays to zero with a time constant ofabout 15 microseconds. For this reason, the digital output of the senseamplifiers must be saved in a latch for later use. The clock pulseoccurs about 2 microseconds after the drive pulse rising edge to do thisdata storage. No data is altered at the trailing edge of the drivepulse, although there is a signal coupled into the column lines.Normally the drive pulses will have their width established such thatthe response caused by the trailing edge of one pulse will not interferewith the desired response from the leading edge of a following pulse.More than one pulse may be high at a time without causing problemsbecause of the 15 microsecond time constant being short in relation tothe drive pulse width.

It is important to note that in the above description, the value of VREFused for the sense amplifier comparison is derived from the input drivepulse and will have an exactly proportionate amplitude. In a similarmanner, the signal from the sense column will depend on the capacitancematrix of the sensing element and the element being sensed. Thiscapacitance matrix produces a division of the drive pulse which dependsonly on physical dimensions and the conductive cylinder location.Therefore the use of a VREF derived from the drive pulse amplitude tocompare with a signal also derived from the drive pulse amplituderesults in a system which to the first order is independent of theactual drive pulse amplitude. This operational feature substantiallyimproves the quality of performance and operating margins for errors. Italso permits the use of the sensing system in different machines withvarious values of drive pulse amplitude, such as 10 volts in one machineand 24 volts in another.

Each of the drive lines for the 16 row by 36 column sensor array has acircuit similar to the circuit schematic diagram shown in FIG. 4. Theexternal drive pulse source can be represented as a voltage source 66with its output going between 0 volts and +10 volts, and having a risetime of 100 nanoseconds and a fall time of 300 nanoseconds. This voltagesource has an output impedance of less than 10 ohms. The drive pulsegoes first to R1, a 1.0K resistor to ground. This causes the drive lineto go to ground if not connected, and puts a minimum load on the drivepulse source for better performance. It then goes through R2, a 100 ohmresistor in series with the array line. This limits the driver currentoutput if a short occurs elsewhere in the circuit. In addition R2 actsin conjunction with C1, a 470 pF capacitor as a low pass filter to limithigh frequency noise from the drive pulse source being put on the driveline. The voltage on C1 then drives the row line in the sensor array. Atthe far end of the sensor array, the row line 61 is terminated in aseries combination of R4, 56 ohms, and C3, 220 pF. This serves toterminate high frequency waveforms in a load similar to thecharacteristic impedance of the trace structure for the drive rings andconnecting traces.

In addition to the above, each row line 61 has connections to bus Athrough a series combination of C2, 22 pF, and R3, a 15K resistor. Thisinjects an impulse of charge into bus A whenever a row line makes atransition. A separate circuit described later uses the bus A signal todetect that a row pulse has occurred. Each row line also has connectionsto bus B through diode D1, a silicon small signal switching diode. Thus,whenever a row line is high, bus B will be pulled up to approximatelythe same voltage. This is used to generate the VREF voltage used in eachsense amplifier 64, corrected in the respective columns of the array,for comparison with the sensor array output signals.

Each row line M in the sensor array has a total capacitance to theremaining circuitry of about 60 pF, for 36 drive rings andinterconnecting traces. The transmission line surge impedance of thisrow line is approximately 56 ohms when constructed on FR4epoxy-fiberglass printed circuit board material of {fraction (1/16)}inch thickness.

FIG. 5 shows the pulse detector 54 and clock generator 56 circuits. Theinput charge pulses caused by the row drive inputs go through a seriesresistor R5, 100 ohms, to the base of transistor Ql, a 2N3904 generalpurpose silicon amplifier. Resistor R8, 2.2K, serves as the loadresistor for Ql to cause it to operate as an amplifier, with resistorR7, 15K, providing feedback. Resistor R6, 4.7K, establishes the biasoperating point so that the collector voltage will be about 4 times thevalue of Vbe for the silicon transistor. At room temperature withtypical parts, this gives a quiescent voltage on the transistorcollector of about +2.8 volts DC. When a +10 volt row line positivetransition occurs, the charge coupled through C2 and R3 previously showncauses the transistor to saturate with its collector at about +0.1 volt.This saturation state remains for 700 nanoseconds, and the collectorthen returns to its quiescent voltage of +2.8 volts with a time constantof 30 nanoseconds. The falling edge of the voltage at the Ql collectoris connected to the A input of the monostable multivibrator U1, causingit to begin outputting a pulse. The other inputs are tied to +5 volts orground as needed since they are not used. The output pulse width is setby the capacitor C4, 330 pF, and resistor R9, 4.7K ohms to a value ofapproximately 2 microseconds. The monostable output pulse is positivegoing at the pin Q, and goes from H to L at pin Qbar. Since the datalatches used later respond to a rising edge and it is desired that theyclock in data when the monostable pulse terminates, the Qbar output isused to drive the clock line to the latches. Capacitor C5, 0.1 uF, is anoise bypass for power to U1 as commonly practiced in the state of theart.

The amplitude reference generator 60 is shown in FIG. 6. This circuittakes the voltage developed on bus B, which is approximately equal tothe row drive pulse, and produces a voltage VREF for use by the columnsense amplifiers. First, capacitor C6, 1.0 nF, bypasses noise impulsesto ground. Then resistors R10 and R11 in series operate as a voltagedivider with adjustable resistor R12 to output a fraction of the bus Bvoltage as VREF. Diode D2, a silicon small signal switching diode, isconnected to the tap between R10 and R11 to prevent its going belowabout +5 volts. In this way, VREF is kept from going to zero between rowpulses, which would cause the sense amplifiers to receive both signaland reference inputs of zero. This prevents chatter and oscillations inthe comparators used for the sense amplifiers.

The sense amplifier 64 and latch circuit 58 are shown in FIG. 7. One ofthe 36 column lines 63 from the sense element array is connected to aresistor R13, 470K, to ground to define the average DC signal level. Italso goes to the + input of a comparator U2, a TLV2352ID, which servesas the sense amplifier. The comparator is a high gain amplifier whichproduces a digital output which is H if the + input is more positivethan the − input, and L for the reverse state. The −input of thecomparator receives the VREF voltage which represents the best level fordiscrimination between sense holes filled by a metal cylinder, and senseholes filled by a dielectric rod. This level is normally set to 35millivolts by resistor R12 in FIG. 6. Power for the comparator is +5volts provided to pin 8, with a 0.1 uF noise bypass capacitor C7. Theground return is pin 4. Output from the comparator is taken out on pin1, which has a pull-up resistor R14, 10K, to +5V. The comparator outputis an open drain stage, so a source of current to pull it up to a logicH level is needed. Output from the comparator also goes to the D inputof the latch U3 (58), a 74AC564 octal inverting edge triggered latch, onpin 2.

The inverting form of the latch is used in this particular applicationso that the latched outputs will go low when a signal is received togive compatibility with other equipment. A non-inverting latch could beused to give similar results. When the clock signal from FIG. 5 makes alow to high transition on pin 11, the latch will store the present valueon its D input and present it at its Qbar output on pin 19. At all othertimes, the value of the Qbar output is unchanged. The output enable barfor the latch on pin 1 is permanently connected to ground so the outputalways stays ON, since this function is not needed. Comparator U2 comesin a package with two sections. The remaining section is used for afunctionally identical circuit not shown here. Latch U3 comes in apackage with 8 sections, and the other sections are used for othercolumns not shown. A total of 18 dual comparator packages and 5 octallatch packages are used to make the circuitry for sensing and latchingthe outputs of 36 columns.

A portion of the sensor element array is shown in FIGS. 8A and 8B. It ismade with elements as shown previously in FIG. 1 arranged in a uniformlyspaced array with a center to center spacing of 0.315 inch. The arrayhas 16 rows of 36 column, with shielding ground traces between each ofthe columns of sense rings to minimize parasitic coupling. The physicalsensing array has the drive circuitry at one end of the drive rows, andthe sense amplifiers located at one end of the columns. The arrangementis done so as to minimize the possibility of stray signals being coupledinto the column sense lines and to minimize the length of traces on thecolumn sense lines.

A preferred form for the cooperating sensing pin 16 previously alludedto will now be described. FIG. 9 shows a cross-section of the pin at anypoint along the constant diameter portion of its length, drawn with amagnification of 20X. The pin is made with a nominally constant diameterof 0.180 inch for use in a sensor hole of 0.200 inch internal diameter.Two special features of the pin's cross-section are flattened sides 70where the mold halves mate together to reduce problems with spuriousmold flash protrusions, and small longitudinal ribs 72 to minimizeabrasion of the cylindrical surface. The longitudinal ribs are sized sothat their outermost surfaces are approximately 10 percent of the totalcircumference. The ribs protrude only 0.010 inch so that they enforce anair gap around the pin, but do not excessively widen the air gap. Equalspacing of the four ribs around the circumference provides support forall possible directions of abrasion.

FIG. 10 is a longitudinal view of the sensing pin 16 with one of thefour longitudinal ribs detailed. At one end of the shaft, a button head76 retains the pin in its equipment, preventing accidental loss of thepin. The pin normally protrudes through a metal sheet with appropriateholes for sliding motion. The other end of the shaft has a rounded shapeto minimize the possibility of tissue damage when pressed against a softsurface. A special molding plastic is used for fabricating the pin thatincorporates both fiberglass for strength and PTFE for built-inlubrication. The plastic has a very low conductivity, and behaves as anexcellent insulator. At a location approximately halfway between theends of the pin, a zone or region 80 of the plastic 0.18 inch long ismade conductive with an evaporated metal coating. Since the sensor beingused is very sensitive to electrostatic fields and draws essentially nocurrent through the pin, the metal coating can be very thin. The sensorwill work correctly for metal layers with sheet resistances of 10,000ohms per square or less. A layer of chromium or aluminum with athickness of only 4 microinches gives a sheet resistance of less than0.5 ohm per square, and is partially transparent to the eye. Forsturdiness, a chrome layer of 10 to 40 microinches is desired, but fromthe above numbers almost any layer thickness would do. The layer couldbe invisible and still be effective. It is important that the pinsurface not intended to have conductive properties should not receiveany evaporated metal, and the transition zone from metal to clear shouldnot extend over more than about 0.02 inch of axial length at each end ofthe metal region.

The longitudinal ribs are now seen to serve the purpose of keepingmotion of the pin in the sensing hole from rubbing the thin metal layeroff of the surface of the pin. Friction will remove metal from the topsof the ribs, but they constitute a small portion of the total metalcircumference. The overall sensor sensitivity should drop by less than10 percent with wear and age.

The plastic pin 16 is injection molded from a mixture of a thermosettingplastic with 15 percent of fiberglass short fibers and 15 percent ofPTFE added to the mixture. The pin is treated as needed to promoteadhesion of the evaporated metal, and placed in a metal evaporationsystem. A system of metal masks in the evaporation system prevents metalcondensation on the pin anywhere except the desired region as marked inFIG. 10. The pin is then ready for use in the electrostatic sensordevice

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications, and variances, which fall within thescope of the appended claims.

1. A system for electrostatically measuring the shape or pattern ofobjects comprising: an array of sensor devices having the capability ofresponding, due to variable electrostatic coupling, to producedifferentiated output signals when a sensor element forming part of eachdevice is present and when it is not, wherein said array includes M rowsand N columns of sensor devices; a source of drive pulses connected tothe M rows; a detector for determining that a pulse has occurred on oneof the M input rows; a clock generator connected to the detector; senseamplifiers in the N columns for sensing the output signals from thesensor devices; and an amplitude reference generator connected to the Mrows for providing a reference voltage such that if the reference valueis exceeded, the respective sense amplifier output is stored.
 2. Asystem as defined in claim 1, wherein the location or position of all ofthose sensor devices, that have the sensor element present so as to forma part of the device, defines the shape or pattern of the objects beingmeasured.
 3. A system as defined in claim 1 wherein the referencevoltage is substantially proportional to an amplitude of the drivepulse.
 4. A system as defined in claim 3, further comprising latchesconnected to the respective sense amplifiers for storing the senseamplifier outputs from all the columns.
 5. A system as defined in claim1, further comprising grounded shield traces between the N columns. 6.The system as defined in claim 1, wherein the array of sensor devicescomprises a first and a second sensor element, further comprising: adielectric substrate material formed in two layers, with a conductingplane between the two layers; a sensing hole which penetrates thedielectric substrate material from its upper surface to its lowersurface, the first sensor element being receivable on said sensing hole;a clearance hole in the conducting plane such that conducting plane doesnot obstruct the sensing hole; and a first and second conducting ringssurrounding the sensing hole on the upper and lower surfaces,respectively, of the dielectric substrate for defining the second sensorelement.
 7. The system as defined in claim 6, further comprising asignal source connected to the first conducting ring and a sensingcircuit connected to the second conducting ring.
 8. The system asdefined in claim 7, wherein the first sensor element is a dielectric pinhaving a conductive region, such that when the pin is present in thesensing hole and a pulse is applied to the first conducting ring, asubstantially large signal is transmitted by the sensing circuitconnected to the second conducting ring.
 9. The system as defined inclaim 6, wherein an inner periphery of the first and second conductingrings are not exposed on the interior surface of the sensing hole.